Control rod drives are employed in nuclear reactors to position control rods with respect to the core of the reactor. Depending upon whether sufficient numbers of control rods are inserted into the core at appropriate locations and whether these rods are completely or partially inserted, the reactor is either completely shut down or its operation is modified to continue at a different level or according to a different power distribution within the core. Fast or emergency insertion of the control rods is referred to by those skilled in the art of nuclear reactors as "scram." Normal insertion or withdrawal of the control rods in the reactor, on the other hand, is called "shimming."
Control rod drives for electromechanically establishing the level of control rods in a reactor for normal operation, optimal power shaping, and economical fuel management have been known for years. One such drive is the subject of a patent application in West Germany by Kraftwerk Union AG. Offenlegungschrift Pat. No. 2442722 contains the application which was laid open to the public on Mar. 25, 1976.
This drive is not sectionable into parts for removal, however, making it necessary for the entire drive to be removed in one piece from the reactor pressure vessel for maintenance, repair, or disassembly. Furthermore, this drive does not employ a differential pressure technique as disclosed herein to prevent inadvertent uncoupling of the control rod drive and the control rod itself during reactor operation or scram. These features, among others, serve to distinguish the instant invention from the apparent prior art.
The invention herein is applicable to boiling water reactors, in which the core is covered with water for cooling, and which are designed to insert control rods from below the pressure vessel containing the core of the reactor. The claims cover concepts and features, however, which can be employed in other kinds of reactors and in linear-motion producing devices not within the field of reactor technology.
For more details with regard to the kinds of nuclear reactors, see pages 5-1 to 5-129 in Energy Technology Handbook (1977) by McGraw-Hill, [Douglas M. Considine, P.E., editor-in-chief]. Additionally, Nuclear Reactor Engineering by Samuel Glasstone and Alexander Sesonske, published in 1981 by Van Nostrand Reinhold Company, and Introduction to Nuclear Engineering by John R. Lamarsh, published in 1977 by Addison-Wesley Publishing Company, are useful references.
The reactor pressure vessel in a boiling water reactor is supported by a massive foundational structure or pedestal, perhaps constructed of concrete. The structure defines an opening or "hatch" which permits the essentially horizontal transportation of the control rod drive between the underside of the pressure vessel and other compartments of the reactor facility.
The underside of the pressure vessel defines a pit of predetermined dimensions and is bounded by the walls of the pedestal supporting the pressure vessel. Numerous control rod drives (their precise number depends upon the size of the reactor core and the number of control rods needed to control criticality of the reactor) extend below the pressure vessel into the pit.
Each control rod drive is suitably coupled to a control rod for contributing to control of the reactor. The core is suitably high in the pressure vessel to permit the control rods to remain within the pressure vessel when fully withdrawn from the core. This requirement makes it necessary for a portion of the control rod drive to traverse the distance between the bottom of the pressure vessel to the vicinity of the core, in order to fully insert a control rod within the core. When this portion of the control rod drive is fully withdrawn from the pressure vessel, it is contained within a pressure tube of the control rod, which is typically about 16 feet long.
In fully hydraulically operated control rod drives, the motive power for scram or partial insertion of control rods into the core is applied through a relatively short package of equipment mounted under the pressure vessel. Electromechanical control rod drives are much longer, because they utilize an electric motor at the lower end of the control rod drive to turn a shaft and spindle to insert the control rod into the reactor core for shimming and which utilize hydraulic drive for scram. In addition, seals and various coupling devices increase the overall length of the control rod drive.
Installation and removal of such a control rod drive is accomplished by translating the aspect of the control rod drive between vertical and horizontal postures. Passage through the hatch in the pedestal is horizontal; however, the control rod drive is vertical when installed. Within the limited space available in the pedestal under the pressure vessel, the control rod drive is turned from one aspect to another. Clearly, with a longer control rod drive this is made more difficult and possibly impossible.
Even when installation and removal of the control rod drive is accomplished within the bounds and dimensions of the pedestal, preventing the uncoupling of the control rod drive from the control rod itself during reactor operation or scram remains a significant challenge.
The electromechanical control rod drive of interest herein "scrams" by applying high pressure directly under the head of a hollow drive piston which carries the control rod. In contrast to some kinds of fully hydraulic control rod drives such as are disclosed in U.S. Pat. Nos. 3,020,887 and 3,020,888 (hereby expressly referred to and incorporated herein), in which the insertion pressure is applied primarily at the foot of the piston (or index tube), the pressure here is applied directly under the head of the piston in the structure discussed with reference to an embodiment of the invention herein.
The uncoupling rod for detaching the control rod drive from the rod itself is designed to traverse the head of the drive piston. In the case of fully hydraulic drives, leakage through the aperture receiving the uncoupling rod is of no great detriment.
However, in the case of the electromechanical control rod drive, excessive leakage through the piston head would prevent the establishment of a scram-suitable pressure. To control leakage, circumferential ridges and depressions are accordingly formed along the sides of the uncoupling rod. The rod itself is enclosed within the head of the aforementioned drive piston by a second smaller piston which is similarly formed with circumferential ridges and depressions, which inter alia help prevent adverse effects due to impurities, particles, and the like which may be found in the water.
To disassemble the control rod, the uncoupling rod is axially translated in an upward direction. Within the tight tolerance required to prevent leakage, water displaced by the movement of the second piston enters or leaves the chamber region by passages provided in the head of the drive piston.
Another challenge is encountered regarding high pressure scram. The pressure under the head of the first piston tends to drive the uncoupling rod upward, causing the control rod and the control rod drive to uncouple. Furthermore, at the end of a stroke, portions of the control rod drive including the piston decelerate so rapidly that the inertia of the uncoupling rod causes the rod to destabilize and potentially separate the control rod from the control rod drive.